Force measuring device



Mam}! 1952 J. B. LA COSTE ETAL 2,589,710

FORCE MEASURING DEVICE Filed June 11, 1948 3 Sheets-Sheet l LUCI EN d.B.LaCOSTE ARNOLD ROMBERG IN V EN TORS BY Mm *fifM ATTORNEYS March 18,1952 J. a. LA COSTE ET AL 2,539,710

FORCE MEASURING DEVICE 5 Sheets-Sheet 2 Filed June 11, 1948LUClENJ.B.LaCOSTE ARNOLD ROMBERG INVENTORS 6. M

ATTORNEYS March 1952 L. J. B. LA cosTE ET AL 2,589,710

FORCE MEASURING DEVICE Filed June 11, 1948 Sheets-Sheet 5 I 'T 4 i T l27 6 am I LUCIEN J-B.L1COSTE 1- I ARNOLD ROMBERG 242 I INVENTORS I I 2wBY f w FIG. 4 7 KM ATTORNEYS Ratenieci Mar. 18, 1952 ()FFICE FORCEMEASURING DEVICE Lucien J. B. La Coste and Arnold Romberg,

Austin, Tex.

Application June 11, 1948, Serial No. 32,386

'7 Claims. 1

This invention relates to improvements in measuring instruments whichmake it possible to obtain measurements when a steady base for theinstrument is not available. It is of particular importance in makingunderwater gravity meter measurements and in making ordinary gravitymeter measurements in marshy areas and near volcanoes.

This application is a continuation-in-part of our copending applicationSerial No. 696,494, filed September 12, 1946. p

The present invention utilizes the basic principles given in the abovementioned co-pending application but includes some refinements that makeit possible to use the invention under less favorable conditions andalso make it possible to obtain data more quickly.

An object of the invention is to obtainan underwater, remote controlledgravity meter with which gravity measurements can be made in roughweather and on soft bottoms.

Another object is to obtain a gravity meter which which gravitymeasurements can be made when the instrument is resting upon a supportwhich undergoes seismic motions or motions similar thereto such assupports undergo in marshy land.

A further object of the invention is to prevent the moving beam in thegravity meter from striking its limiting stops when the gravity metersupport is accelerated.

A still further object is to counterbalance large accelerations of thegravity meter support by introducing additional counterbalancingaccelerations.

An additional object is to prevent such additional counterbalancingaccelerations from introducing errors into the gravity measurements.

A further object is to damp out large initial kinetic energies that thegravity meter beam might have when it is first released or which itmight acquire when the meter is badly jarred or which it ordinarily haswhen the beam is bouncing on the stops because of motion of the gravitymeter support.

A still further object is to provide means for the operator to partiallycounterbalance such initial kinetic energies in order to obtain readingsmore quickly.

An additional object is to improve the operation of the invention asdescribed in our copending application Serial No. 696,494.

The foregoing objects are primary objects and together with otherobjects will become more apparent by reference to the following descrip-"tion and accompanying drawings in which:

Fig. 1 is a side elevational view, partly in section, showing thepreferred form of the invention;

Fig. 2 is a wiring diagram of the main circuit of the inventionincluding the integrating circuit and the servo circuit;

Fig. 3 is a wiring diagram of the circuit for quickly applyingsubstantial torques to the gravity responsive beam of the meter;

Fig. 4 is a wiring diagram of the circuit for indicating the verticalposition of the gravity meter heater box with respect to the containerwhich encloses it.

In view of the eiforts to explore underwater and marshy areas attemptshave been made to use ordinary gravity meters in underwater geophysicalexploration by submerging them and allowing the meter to rest on thebottom. In these attempts, however, satisfactory results have beenobtained only on calm days and only on bottoms which were relativelyfirm.

The importance of making a gravity meter for underwater use that can .beemployed on soft bottoms can hardly be overemphasized because a completegravity map cannot be obtained otherwise in many regions. Furthermore,in view of the high cost of underwater gravity surveys it is extremelydesirable that underwater gravity meters be capable of operation inweather that is not calm.

The failure of underwater gravity meters, heretofore known, to operatein rough weather or on soft bottoms is due to two difliculties:

(1) Water currents disturb the gravity meters; and

(2) There are movements of the bottom upon which this instrument restsduring the taking of readings. The first of these difficulties, namely,water currents, can be sufiiciently reduced by streamlining the gravitymeter and making it dense compared to the water or by various othermeans.

The second difliculty, namely, the motion of the bottom, can be resolvedin rotations, horizontal translations, and vertical translations. Therotations are small and do not cause substantial trouble.

The horizontal translations are sometimes large enough to cause themoving system of a gravity meter to strike the stops provided in themeter housing for limiting the motion of the moving system. This motionmust be kept small to avoid errors due to hysteresis in the gravitymeter spring. However, considerable damping for relative horizontalmotion between the gravity meter housing and moving system can beprovided without interfering with the operation of the gravity meter.This will prevent horizontal translations from causing the movablesystem to engage the stops.

Vertical translations however are also large enough to cause the beam ofthe moving system to strike the stops but in this case the verticaldamping in the gravity meter cannot be increased sufficiently to preventthe beam from hitting the stops without seriously afiecting theoperation of the gravity meter. Increased vertical damping increases thereading time by a prohibitive amount. Nor is it possible in general toprevent the beam from hitting the vertical stops by increasing thespacing of the stops without introducing prohibitive errors due tohysteresis in the gravity meter spring. Hysteresis errors can be keptsmall only by preventing the spring from changing its lengthappreciably.

If the moving system of a gravity meter can be prevented from hittingthe stops, a reading can be obtained by averaging the position of themovable system or gravity meter beam. A satisfactory electrical averagercan be made for a good gravity meter. A diagram of such an averager orintegrator is included in Fig. 2 of this disclosure. The real problemtherefore is to obtain a device which will prevent the gravity meterbeam from hitting the vertical motion stops when the meter is subjectedto vertical accelerations.

The present invention comprehends a device for temporarily displacingthe frame which supports the moving system so that, during movement ofthe system, it will not strike the frame or steps provided. Thedisplacing of the frame merely moves it out of the path of the movingsystem temporarily until the moving system starts back on its reverseswing and the frame is then returned to its initial position.

Such displacement of the frame avoids interference with the movingsystem so it does not affect the average beam position. Such movement ofthe frame prevents vertical translations of the ground from causing thebeam to strike the stops by applying counterbalancing translations whenthe beam is near the stops. Obviously the counterbalancing translationsor displacements of the frame will introduce errors into the measurementof gravity unless their average acceleration during the time of taking areading is negligible. This condition is met by introducing suitableadditional translations While the gravity meter beam is not in danger ofstriking either the top or bottom stops on the frame. The method ofdoing this will be made clear by a study of the device inself.

In Fig. 1 the submersible portion of a remote controlled gravity meteris shown resting on the bottom 2 below a body of water 3., The

submersible portion 1 is shown as including a watertight container llfixed to leveling jacks 5 whose extensible members 6 rest on the bottom2 of the body of water. The detailed construc" tion of such jacks is tobe found in our co-pending application Serial No. 678,204 filed June 21,1946,,

for Leveling Device. The container i is leveled by varying the positionof the extension members 6.

In the container 4 is mounted a gravity meter 7 of the general typeshown in our Patents Nos.

not necessarily those for the best gravity meter.

design but instead are given for purposes of 4 illustration sincedetails of this nature are not a part of this invention. Gravity meterdetails are given in our above patents.

The gravity meter l includes a top plate 8, and outer housing 9, whichare suitably secured together. The inner housing ii) is fixed to the topplate 8 through the heat insulating blocks l I. The gravity responsivebeam 52' is suspended by the main spring 18 and the pair (one behind theother) of springs 14. The spring I3 is clamped to the beam 52 by theclamp IE on the beam. The spring 13 is supported from the inner housingi9 through the block it, the leaf spring ll, which is clamped to blockit, and the clamp IE on the leaf spring. The pair of springs are clampedto the blocks 19 on the inner housing iii and their opposite ends areclamped to the beam l2 by the clamps 23. It is therefore clear that thebeam 52 will rotate about a horizontal axis under the influence ofchanges in gravity.

Rotation of the beam i2 is limited by the lower stop 2i and the upperstop 22, both of which are fixed with respect to the inner housing H3 insuch a manner that the beam is limited in its movement to an extent thatthe spring l3 will not be subjected to appreciable hysteresis. The forceof gravity on the beam is balanced by vertical adjustment of the clampI8 to which the main spring i3 is clamped.

This vertical adjustment is produced by turning the crank 23 of theself-synchronous generator 25 which is electrically connected throughcable 25 to the self-synchronous motor 26. The motor 25 turns as manyrevolutions as the generator 2 3. The shaft 2'." of the motor 2%transmits rotation to the screw 28 through the gears 29, 39, Si, and 32.The lower end of the screw 28 presses against the leaf spring ll. As thescrew 28 moves downwardly the clamp IE on the leaf spring movesdownwardly a smaller amount. The tension on the main spring 13 andtherefore the equilibrium position of the beam [2 can thus be adjustedby suitable rotation of the crank 23 of the self-synchronous generator26 on the boat 32.

A photoelectric cell system is used to give remote indications of theposition of beam l2. The system includes an electric lamp 34' clamped inthe bracket 35, which is fixed to the outer housing 9, and the lamp 202clamped in the bracket it which is also fixed to the housing 9. Nosource of potential for lighting lamps 34, and 202 is shown in Fig. l inorder to avoid complicating the drawing, but in .practice the two lampsare connected in parallel to a battery on the boat 33. The two lamps 32and 262 are matched so that battery fluctuations afiect the amounts oflight emitted by the two lamps, substantially in the same ratio.

Lamp 202 is used as a standard lamp. A definite fraction of the lightemitted by it falls upon the cathode M of the double photoelectric cell#53, which is fixed to the bracket 44.

Lamp 34 illuminates the slit 35 in the bracket 35. Light from theilluminated slit strikes the mirror 31 which is also fixed to thebracket 35; the light is reflected to pass through the converging lens38 mounted on the inner housing Ill. The light then is reflected fromthe mirror 39 fixed to the beam l2 and passes through the lens 38 again.The lens causes the light to converge and form an image 40 of the slit36.

When the beam 12 is at its highest position the image 40 is formed inthe slit 2B! in the bracket 44, and therefore the lightv from it fallson the cathode 42 of the double photoelectric cell 43. But as the beammoves down part of the image 40 misses the slit 20| thereby reducing theamount of light thatv strikes the cathode 42. Generally the fraction ofthe total light from the standard lamp 202 that is allowed to fall onthe cathode 4| is made equal to the amount of light from lamp 34 thatfalls on cathode 42 when the beam is at its mid-position or referenceposition. As the beam moves up more light will fall on cathode 42 thanon cathode 4| and as it moves down less light will fall on cathode 42than on cathode 4|. The photoelectric cell 43 is electrically connectedto the amplifier 45 through the cable 45. The amplifier gives a voltageoutput proportional to the difference in the amount of light falling onthe two cathodes. Furthermore, the voltage output has one sign when beamI2 is near the top stop 22 and the other sign when the beam is near thebottom stop 2|. Amplifiers capable of satisfying these conditions arewell known in the art.

A diagram of such an amplifier is included in Fig. 2. The operation ofthe amplifier is as follows. The two cathodes 4| and 42 of the twinphotoelectric cell 43 are connected respectively to the grids 203 and204 of the approximately matched vacuum tubes 205 and 206. The grids 203and 204 are also connected to the negative terminal 208 of the B battery20'! through the approximately equal high resistances 209 and 2|0respectively. The negative terminal 208 is connected to the cathodes 2Hand 2| 2 of tubes 205 and 206 through the cathode biasing resistor 2| 3.The positive terminal 2| 4 of B battery 20'! is connected to the anodes2| 5 and 2|6 of the twin photoelectric cell 43. The positive terminal2|4 is also connected to the plates 2|? and 2|8 of tubes 205 and 206through the plate resistors 2|9 and 220 respectively. These resistorshave variable taps 22| and 222.

The taps are adjusted so that when there is no light on the twophotoelectric cell cathodes 4| and 42 the voltage between the taps 22|and 222 is approximately zero. This adjustment can be made accurately tozero but it cannot be maintained this way without further adjustmentbecause of aging of the vacuum tubes 205 and 203 and because of impactsto which the tubes may be subjected. Therefore it will merely be assumedthat the adjustment is made to make the voltage between taps 22| and 222approximately zero.

The actual value of this voltage at any time will be used as a referencevoltage in determining the reference position of the beam I2.

Since the two tubes 205 and 203 are approximately matched, the voltagebetween taps 22| and 222 will remain approximately zero when the twophotoelectric cell cathodes are equally illuminated regardless of theintensity of the illumination. The zero or reference position of beam l2will be taken as the beam position which produces the same voltagebetween taps 22| and 222 regardless of whether the photoelectric cellcathodes are illuminated or not.

It should be noted that when the beam 2 moves upwardly, more light fallson cathode 42 which makes tap 22| more positive than tap 222 and whenthe beam moves downwardly less light falls on cathode 42 and tap 222becomes more positive than 22|. The function of vacuum tube 240 will beexplained later in connection with the servo amplifier 48.

Connections from taps 22| and 222 are made to the averager or integrator41 through the cable 49. The connections are made to the inte grating oraveraging circuit consisting of the resistors 226 and 22'! and thecondenser 228 which are connected as shown. The resistors 226 and. 221are made large compared to resistors 2|9 and 220 so as not to affectappreciably the operation of the amplifier just described. A good valuefor the time constant of the integrating circuit is about ten seconds.The averaged voltage appears across the condenser 228 and is appliedbetween the grid and cathode of vacuum tube 229 through the cathodebiasing resistor 230. Vacuum tube 23| is connected to tube 229 to act asa phase inverter in order to make it possible to obtain approximatelyzero voltage on the galvanometer 5| when the voltage across condenser228 is zero. Of course this requires suitable adjustment of the taps 233and 234 on the plate resistors 235 and 236.

The reference position of galvanometer 5| corresponding to the referenceposition of beam i2 is, of course, its reading when the photoelectriccell cathodes 4| and 42 are not illuminated and electric transients inthe averaging circuit have been allowed time to die out. The deflectionof the galvanometer 5| from this reference position when thephotoelectric cell cathodes are illuminated then indicates the magnitudeand direction of the averaged deflections of the gravity meter beam |2from its predetermined zero or reference position.

The parts of the underwater gravity meter which have been described upto this point are sufficient to enable measurements of gravity to bemade when there are not appreciable disturbances on the bottom of thebody of water, that is, when the disturbances are not large enough tocause the beam l2 to strike the stops 2| and 22 very often. Readings aretaken by merely rotating the crank 23 until the meter 5| on the averagerindicates its reference reading and noting the total angulardisplacement in revolutions and fractions thereof of the crank from apredetermined reference position. The difference in gravity at twostations is proportional to the difference in angular displacements ofthe crank required to obtain readings at the two stations.

The present invention relates particularly to the portions of theunderwater gravity meter which eliminate the bumping of the beam on thestops and will now be described.

The top plate 8 of the frame of the gravity meter 1 has two verticalrods 52 fixed to it. These rods can slide freely in holes 53 in brackets54 which are fixed to the watertight container 4. The outer housing, orframe 9, is thereby capable of being translated or displaced verticallyrelative to the container 4 but not horizontally.

Predetermined amounts of vertical translation are made possible by meansof the following construction. The Bakelite (or other heat insulatingmaterial) piece 55 is fixed to the container 4. The ball bearing 60 ispressed into the hole 6| in the piece 55. The shaft 62 is pressed intothe inner race of the ball bearing 60 and has a shoulder 63 which fitsagainst the top of the inner race. The ball bearing thus takes downwardthrust exerted by the shaft 62.

A second bearing 64 for shaft 62 is provided by the plate 65 which isfixed to the piece 55 through the Bakelite blocks 65. A threaded portion61 is provided on the upper end of shaft 62. This threaded portionengages a nut 68 whichis 'to the inner housing If}.

fixed to the outer housing 9 through the Bakelite spacer 69.. A gear Tilis fixed to. shaft 82.

This gear engages gear it which is fixed to countershaft 72. The bevelgear i3 is also fixed to countershaft l2 and engages the bevel gear '54i which is fixed to the shaft '55 of the reversible electric motor 75.The electric motor is fixed to plate 55 and is controlled by the servoamplifier 48 through the cable it. From the preceding description it canbe seen that when the 3; motor shaft 75 rotates, the outer housing 9; is

age between the beam l2 and the lower condenser plate 244. The circuitfor applying the voltage is shown in Fig. 3.

Switch 246 on the panel of unit 43 is used to apply the voltage whendesired. Spring 28l holds switch 246 normally in the up position so thatthere is no voltage between the beam I2 and condenser plate 244, exceptwhen switch 245 is held down. The purpose of applying this voltage is toeffect a downward electrostatic force to the beam l2 in order to reducelarge upward velocities that the beam might haveacquired from bouncingon the stops because of earth motion.

The condenser plate 248 is used to apply a similar upward force. Plate243 is secured to the insulating block 249 which is in turn secured toarm 259. This arm is in turn secured to the inner housing id. Thecondenser plate 248 is also electrically connected to a conductor in themulticonductor cable ll. Voltage between condenser plate 2 58 and beami2 is controlled by switch 241 as shown in Fig. 3. Spring 282 normallyholds the switch 24? in the up position in Fig. 3 thus keeping condenserplate 248 and beam l2 at the same potential except when the switch isoperated.

The cams l8 and 239 are fixed to shaft 62 and v operate respectively thelimit switches 19 and 240'. The lower limit switch 19 prevents the motor76 from lowering the housing 9 below a certain limiting position in thewatertight container 4. Similarly the upper limit switch 240' presentsthe motor 16 from raising the housing 9 above a certain limitingposition in the container 4. These cams and limit switches are alsoshown in Fig. 2 and their operation will be given in more detailhereafter.

. spring 252 which is screwed to the conducting 7 gear '10. Electricalcontact with the conducting gear ill is made by the second leaf spring243 which is fixed'to the Bakelite block 55, and which is electricallyconnected to one of the wires in the multiconductor cable 86.

Other wires in cable 86 are. soldered to; the. two; ends. of thepotentiometer winding 24!. The function ofthis potentiometer assemblyis. to. give. a visual, indication on. meter 244" of the verticalposition of housing 9 with respect to the, watertight container 4... Thepotentiometer circuit is shown in Fig. 4; its operation of meter 244"will be. given in detail later.

The servoamplifier 48 receives its inputs from the beam positionamplifier c5 and from switches 19,. 2'46, 83, and 238;. the. output of.the servoamplifier is supplied to the motor 16 which controlsthevertical translation of housing 9 with respect to container 4. Theoperations which the servoamplifier causes the motor to perform are as;follows:

(1) When housing 9. is approximately at the midpoint of its verticalmotion with respect to container 4, the motor 16 moves housing 9 upwardwhen beam l2 approaches upper stop 22 closely, and the motor moveshousing 9 downward when beam I2 approaches lower stop 2! closely. Themotor does not operate when beam 12 is not near either stop. In this waythe motor prevents beam l2 from striking either stop.

(2) When the housing 9 is appreciably below the midpoint of its verticalmotion. with respect to container 4, motor 16: acts to keep beam l2 nearbut not touching the lower stop 21. The result of keeping beam 12 nearthe lower stop instead of allowing it to move toward the. other stop asin case (1) is to raise housing 9, or to bring it back toward itsvmidpoint with respect to container 4, whenever the beam is not in dangerof. striking a stop.

(3) When the housing 9 is appreciably above its midpoint, the motor l6.acts to keep beam [2 near but not touching the upper stop 22. The resultof keeping the beam near the upper stop is to lower the housing 9., orto. bring it back toward its midpoint with respect to container 4whenever the beam is not in danger of striking a stop.

The way in which the servoamplifier produces these results can beunderstood from a study of the circuit diagram of the: servoamplifier,which is shown in. Fig. 2.

In the previous. discussion of. the operation of .the amplifier 45 ofFig. 2 the. following points were explained. When the beam [2 is at itsreference position, approximately equal amounts of light fall on the twocathodes 4| and 42 of the twin photoelectric cell 43, and the voltagedrop from point 223 to 22.! is approximately equal to the voltage dropfrom point 223 to 222. As beam l2 moves upwardly the amount of lightfalling on cathode 42 increases and the potential of 222 decreases withrespect to that of 223. Similarly when the beam moves downwardly, theamount of light falling on cathode 32 decreases and the potential of 222increases with respect to that of 223.

It is also to be noted that even when no light falls on thephotoelectric cell 43 there is still a considerable voltage drop betweenpoint 223 and 22l or 222. In order to partially or'completely balanceout these voltage drops occurring when the photoelectric cell is notilluminated, tube 250 is used. The tap 224 on the plate resistor 24! 'ofthis tube is adjusted so that when no light 'falls on the photoelectriccell the potential of point 224 is about the same as that of point 221With this adjustment it can be seen that when light is allowed to fallon the photoelectric cell the amount of light falling on cathode 6,2 is

approximately proportional to the voltage between 224 and 222 and theamount of light falling on cathode 4! is approximately proportional tothe voltage between 224 and 22!. The exactness of this proportionalityis somewhat reduced by the effect of the cathode resistor 2!3 but thiseffect can be largely compensated for by properly adjusting tap 224. Itis not necessary that exact proportionality exist.

The purpose of obtaining two voltages which are approximatelyproportional to the amounts of light falling on the two cathodes is asfollows. It is desired to find a voltage which is zero when the beam 2is at a given position or in other words it is desired to find a voltagewhich is zero when a definite ratio exists between the amounts of lightfalling on the two cathodes regardless of the absolute magnitudes of thelight intensities. If two voltages are available which are proportionalto the amounts of light falling on the two cathodes, it is merelynecessary to compare the correct fraction of one voltage with the othervoltage in order to obtain the required zero voltage. The fractions ofthe voltages can be obtained with potentiometers. In Fig. 2 thepotentiometers 242 and 243' are used to obtain two different fractionsof the voltage 224-22! to compare with the voltage 224222 in order todetermine two different positions of beam I2. Similarly thepotentiometers 24 and 245 are used to determine two different fractionsof the voltage 224-222 to compare with the voltage 224-22! in order todetermine two more positions of beam !2. The function of resistors 246'and 24'! and condensers 246 and 249 is to differentiate or filter thevoltages under consideration in order to improve the stability andresponse of the servo system being described. The use of suchdifferentiating orfiltering networks is well known in the servomechanismart; see for example chapter VII in Servomechanism Fundamentals byLauer, Lesniok, and Matson (published by McGraW-Hill).

The potentiometers 242, 243' 244, and 245' are connected to the contactsof the two double pole double throw relays 250 and 25! as shown in orderthat on the two tongues of either relay there will appear a comparisonof one of the two voltages 22422! or 224-422 (after some filtering) witha fraction of the other. When either relay is energized, a differentcomparison is made or in other words a different beam position isdetermined by the vanishing of the voltage between the relay tongues.Each relay can pick out one of two beam positions.

Relay 25!! picks out one of the two beam positions at which the motor ibegins to raise the housing 9 with respect to container 4. Similarlyrelay picks out one of the two beam positions at which the motor 76begins to lower housing 9 with respect to container 4.

It has been mentioned that when housing 9 is approximately at themidpoint of its vertical motion with respect to container 4, motor '76operation of the motor 16. Contact 252 of potentiometer 242' is adjustedto cause housing 9 to move downwardly when beam !2 is near the lowerstop, thus preventing the beam from strik- 1 ing the lower stop.Similarly contact 25! of potentiometer 245' is adjusted to cause housing9 to move upwardly when beam i2 is near the upper stop. When the beam isnot near either beam got close to the stop, it will continue to Howeverit is now required do so in this case. to move housing 9 upwardly ifbeam !2 gets very far away from the lower stop. Contact 253 ofpotentiometer 243' is adjusted to produce this result.

The third condition to be satisfied is a follows.

When housing 9 is appreciably above the midpoint of its vertical motionwith respect to container 4, motor 16 should keep the beam !2 near butnot touching the upper stop. When housing 9 is appreciably above itsmidpoint, cam 23'! closes switch 238 while switch 83 is open. Relay 25!is now energized While relay 256' is not. The

potentiometer 245' on relay 259 is already properly adjusted to preventthe beam from getting too close to the upper stop. Contact 254 onpotentiometer 244' is then adjusted to move housing 9 down whenever beam!2 gets far from the upper stop, thus keeping the beam from getting toofar from the upper stop.

The way in which the voltage between the tongues of arelay operatesmotor 16 is now explained. Consider relay 258'. The voltage betweenitstongues 290 and 29! is applied between the grid 255 and cathode 256of tube 25'! through the cathode resistor 258. Of course the connectionsmust be made with the proper polarity to cause the motor 16 to move whenthe beam approaches the upper stop rather than when it moves away fromthe upper stop. Tube 259 is connected to tube 25'! as a phase inverter.

Resistor 260 is connected as shown to give the proper bias to the gridof the'next tube 26!.

The plates 283 and 284 of tubes 25'! and 259 are connected to thecathode 26'! and grid 293 respectively of tube 26! which acts as aresistance in the phase shifting circuit composed of tube 26!, condenser262, and of course the transformer winding 263 of transformer 265.

The primary 264 of the transformer 265 is energized by the A. C. source266 which also supplies the'energy As the signal on-tube which drivesmotor 16. I 26! is varied the plate resistance of the tube changesthereby changing the phase of the cur- The use of rent in the ,phaseshifting circuit. phase shifting circuits to drive thyratrons in Theoutput of the phase shifting circuit is taken from the cathode 26! andthe center tap 268 of transformer winding 263. This output is applied tothe primary of the isolatingtransformer 269. The output of transformer269 is 1 applied to the cathode 285 and control grid 286 of:theathyratron 210 through the grid resistor 21!.

The suppressor grid 294 of the. thyratron 2'50 is connected to thecathode 285. The plate 281 of thyratron 270 is connected to one side ofthe A. C- source 266 and the cathode 285 is connected to theother sideof the A. C. source through the permanent magnet motor 15. The operationof the branch of the circuit for reverse operation ofmotor 16 is similarexcept that the thyratron 212i is. connected with opposite polarity inorder to obtain the reversed rotation. The thyratron 212' need not beconnected. with opposite polarity if. a. split series motor be used. Apermanent magnet motor" is used in a currently existing sys tem onlybecause. such motors were more readily available. Suitable thyratrontubes are 205Gs. 6SL7' tubes can be used everywhere else in the circuit-While specific tube types are indicated to aid; the skiller'artisan inpracticing the invention, it is not intended that the invention shall beconfined thereto as any tube having the desired characteristic may beused.

The polarity of the isolation transformers 269 and. 269' must beproperly chosen to cause: the

response of motor 16 to be proportional to the signals on the tongues ofrelays 250 andzt I. The opposite polarity of the transformer connectionsgives: on'or off control of the motor 15 which greatly impairs the servooperation. See p; 285 in Principles of Electron. Tubes by H. J-. Reich(McGraw Hill).

Cam 239 prevents housing 9 from being translated too. far above itsmid-position by opening the circuit which. supplies the motor with enery to raise: the housing 9. Similarly cam 13 prevents housing 9 frombeing moved too far below its mid-position by opening the circuit whichsupplies motor 16 with energy to lower the housing.

It has been mentioned. that the potentiometer 24! transmits to thegalvanometer 244" an indicationof the. relative position of housing 8 inthe container 4. The Wheatstone bridge circuit for accomplishing-thisresult is shown in Fig. 4-. As housing 9 is translated vertically thepotentiometer arm 242, which is fixed to shaft 62, moves along thepotentiometer winding 24!. This varies the resistances of the branches213 and 214' of the bridge and thereby produces a correspondingvariation in the reading: of the galvanometer 244".

The method of operation of the invention when a measurement of gravityis made can now be considered. To. be specific the following assumptionswill be made:

(.1') The gravitational torque on beam !2 is considerably greater thanthat exerted by spring l3.

(2) At the time the observation. is begun the beam: [2: has aconsiderable upward velocity referred to: coordinates fixed. in space.

3) The ground on which container 4 is resting has a vertical oscillatorymotion but the acceleration of this motion averaged over periods as longas'a m'inute or two is practically zero. This earth motion iswhat isactually found on the bottom of' the ocean when the bottom is soft.

Under the preceding conditions the average velocity of beam l2 withrespect to container 4 will: also be'consid'erable and in an upwarddirection, unless it strikes a stop, until the assumed unbalanced:gravitational force reduces the Velocity to zero; As has been explainedthe servomotor 16 will: translate housing 9 verticallywith respect tocontainer 4-. to prevent the beam l2 from striking the stops. as long asthe required translation does not exceed givenv limits. The servo.merely translates housing 9 to keep up roughly with the. motion of beaml2. The question then is, whether the unbalanced gravitational forcewill be sufiicient to reduce the initial upward velocity to. zero beforethe limits of translation of housing 9 are reached.

It often happens that the limits will be reached first. If thi is thecase, the. operator is warned that the housing 9 is approaching itsupper limit by the reading on galvanometer 244", which indicates therelative position of housing 9 in container 4. The operator then pressesswitch 246 for an instant. As has been explained previously thisoperation applies a considerable downward force to the beam I2; whichreduces its upward velocity. If necessary the operator repeatedlypresses switch 246 to stop the average motion of galvanometer 244"toward the point corresponding to the-translation limit of housing 9. Itis important that the operator does not press switch 268 so much thatthe average reading of galvanometer 244" moves. in the oppositedirection with appreciable velocity. Such overcontrolling corresponds togiving the beam an appreciable velocity in the downward direction,which. is no improvement over the preceding case. It is not difficul't'to avoid this overcontrolling.

We will assume that the operator succeeds in approximately Stopping. theaverage motion of galvanometer 244". The operator then discontinuesoperating switch 246 and awaits the results: of any unbalance. betweenthe gravitational force on beam l2 and the force exerted by spring l3.Sincewe have assumed that the spring force is the smaller force, thebeam [2 will experience a downward acceleration. This will cause it tomove downwardly and, because of the operation of the servo, to make thehousing 9 move downwardly with respect to container 4 on the average.This condition is indicated by motion of galvanometer 244" in thedirection corresponding to downward motion. If the gravitational andspring forces are considerably unbalanced, as has .been assumed, theoperator soon has to press switch 241. occasionally in order to stopmotion of galvanometer 244" toward the point indicating the lower limitof translation of housing 9 with respect to container 4.

We will assume that the operator succeeds in approximately stopping theaverage motion of galvanometer 244 or even in giving it. a slightaverage motion inv the opposite direction. We see however that becauseof the unbalanced downward force on beam l2,. the beam will always againacquire a downward velocity which will cause galvanometer 244" to movetoward the position indicating the lower translation limit. This.condition requires repeated use of switch 24.! and no. further use ofswitch. 246. This repeated need. for the use of only switch 241 is adefinite indication that the tension in spring I3 is too small. the useof only switch. 246 is a definite indication that the tension in springvI3 is too. large.

In accordance with these criteria the operator increases the tension onspring l3 by properly op.- erating crank 23 on the generator 24. Again.the operator determines which if any of the switches 246 or 241 it isnecessary to use repeatedly. Again he makes corresponding adjustments byturning crank 23. This process is repeated until he finds a setting ofthe self-synchronous generator 24 at which it Similarly thev repeatedneed for self -synchronous 1 13 or 241 to prevent housing 9 fromreaching either of its limiting positions. The fact that if theadjustment is close enough to correct, the housing 9 will not reacheither end of its range will be discussed later.

The operator then reads galvanometer which indicates the averagedposition of beam I2 in housing 9. If this galvanometer does not indicatethat the beam is at its predetermined reference position, the operatormerely adjusts crank 23 to bring galvanometer 5| to the desiredgalvanometer indication. The desired indication is that obtained when nolight falls on the cathodes of the photoelectric cell 43, as has beenmentioned. The operator then records the total angular displacement inrevolutions and fractions thereof of the crank 23 from a predeterminedreference position. The difference in gravity at two stations isproportional to the difference in angular displacements of crank 23required to obtain readings at the two stations.

It will now be shown that if the setting of the self-synchronousgenerator 24 is close enough to correct, housing 9 will not reach eitherend of its range, assuming of course that the beam does not have a highinitial velocity. Let us assume that the tension of spring I 3 is suchthatif there were no earth motion the beam I2 would come to restsomewhat below its previously determined ref-- i lated by servomotor 16in such a way as to keep the beam I2 near the upper stop 22. Howeversuch a position of the beam produces a spring torque on the beam lessthan the gravitational torque on it because it has been assumed that theequilibrium position of the beam is slightly below the reference point.Therefore the only way that the beam I2 can be kept near the upper stopis for motor IE to give housing 9 an average downward acceleration withrespect to container 4. This average downward acceleration will bringhousing 9 toward the midpoint of its translatory range.

Let us now assume that housing 9 is appreciably below the midpoint ofits translatory range. It has been previously explained that under theseconditions housing 9 is translated by servomotor T6 in such a way as tokeep the beam I2 near the lower stop 2I. Such a position of the beamproduces a spring torque on the beam greater than the gravitationaltorque on it because it has been assumed that the equilibrium positionof the beam is only slightly below the reference point. Therefore, theonly way that the beam I2 can be kept near the lower stop is for motor76 to give housing 9 an average upward acceleration with respect tocontainer 4. This average upward acceleration will bring housing 9toward the midpoint of its translatory range.

We thus see that under the assumed conditions whenever housing 9 is notnear its midposition with respect to container 4, the servomotoroperates to bring it back to the midposition. Furthermore the viscousdamping, which is always present in a gravity meter, between beam I2 andhousing 9 will tend to make the average Velocity of beam l2 the same asthat of housing 9. The way in which this takes place is explained indetail in our copending application, Serial No.

696,494 for the case in which the beam is balanced, the ground isstationary and the beam does not have enough initial velocity to causehousing 9 to reach its translational limits. When there is earth motionwe can therefore expect that the vertical oscillations of housing 9 incontainer 4 will take place about the midpoint of the allowable range ofmotion and will have an amplitude not very diiferent from the amplitudeof the earth motion.

A consequence of the preceding conditions is that there will be noappreciable average acceleration of housing 9 with respect to container4. Furthermore since we have assumed (the actual case) that the groundon which container 4 rests has no appreciable average acceleration withrespect to coordinates fixed in space, the housing 9 which contains thegravity meter has no appreciable average acceleration with respect tocoordinates fixed in space. The averaged indications of gravity aretherefore correct unless the beam strikes the stops, which we have seenthe servo prevents.

The invention has been described in connection with a null readinggravity meter rather than with a deflection type of instrument.Obviously it could be used with a deflection type instrument. In thiscase the reading would be obtained from the averaging galvanometer 5!rather than from the angular displacement of the crank 23 required tomake the galvanometer give its reference reading.

It is also obvious that the present form of the invention can be appliedto other forms of gravity meters, or other force measuring instruments,just as well as the earlier form described in our copending application,Serial No. 696,494. This copending application described a modificationof the earlier form of the invention to a torsion type of gravity meter.

It should be also noted that the two condensers 244 and 248 are notactually essential to the operation of the present form of theinvention. These two condensers are used merely to make possible thequick application of substantial torques to the beam I2. It is possibleto do this with the selsyn 24 by rotating its crank rapidly. Condenserswere used in the preferred form of the invention because they requiredless manip-' ulation on the part of the operator.

The present form of the invention has the following advantages over theforms described in our copending application Serial No. 696,494:

(1) It gives readings more quickly because in the present form housing 9can be prevented from reaching its limits of translation in container 4.This is accomplished by the application of substantial torques to thebeam I2 in accordance with the indications of the position of housing 9as indicated accurately on galvanometer 244".

(2) The present form of the invention can be operated when the groundmotion is too violent to permit the operation of the earlier form. Thisis possible because the servo in the present form has a better responsethan the previous servo and thereby can prevent the beam from strikingthe stops under more adverse conditions.

Broadly, this invention comprehends the application of controlledaccelerations of elements of measuring instruments in order to enableprocurement of accurate measurements under adverse conditions.

The invention claimed is: 1. A. force measuring instrument comprising, a

support, a moving system mounted thereon, said moving system includin amember subjected to the force to be measured whereby the system moves inresponse to variations in such force, means on said support for limitingthe movement of the moving system relative to the support, means fordisplacing said support to avoid engagement of the moving system withsaid first mentioned means, means for applying a force to said systemfor accelerating the system in a direction to limit the requireddisplacement of the support, and additional means for averag ing theposition of the moving system with respect to the support,

' 2. A force measuring instrument comprising, a support, a moving systemmounted thereon, said moving system including a member subjected to theforce to be measured whereby the system moves in response to variationsin such force, means on said support for limiting the movement of themoving system relative to the support, means for displacing saidsupport, said means being operable to avoid interengagement of thesystem with said first mentioned means and to return said support to apredetermined reference position in its range of movement when themoving system is in the midportion of its range of movement relative tosaid first mentioned means, means for applying a force to said systemfor accelerating the system in a direction to limit the requireddisplacement of the support, and additional means for averaging theposition of the moving system with respect to the support.

3. A force measuring instrument comprising, a support, a moving systemmounted thereon, said moving system including a member subjected to theforce to be measured whereby the system moves in response to variationsin such force, stops on said support for limiting the movement of themoving system, means for displacing said support to avoid engagement ofthe moving system with said stops, and means for applying a force tosaid system for accelerating the system in a direction to limit therequired displacement of the support.

4. A gravity meter including, a support, a mass mounted thereon formovement in response to variations in the force of gravity, means onsaid support for limiting the movement of the mass relative to thesupport, means for displacing said support to avoid engagement of themass with said first mentioned means, and means for applying a force tothe mass for accelerating the mass in a direction to limit the requireddisplacement of the support.

5. A gravity meter including, a support, a

mass mounted thereon for movement in responseto variations in the forceof gravity, means on said support for limiting the movement of the massrelative to the support, means for displac ing said support to avoidengagement of the mass with said first mentioned means, means forapplying a force to the mass for accelerating the mass in a direction tolimit the required displacement of the support, and additional means foraveraging the position of the mass with respect to the-support.

6. A gravity meter including, a support, a mass mounted thereon formovement in response to variations in the force of gravity, means onsaid support for limiting the movement of the mass relative to thesupport, means for displacing said support, said means being operable toavoid interengagement of the mass with said first mentioned means and toreturn said support to a predetermined reference position in its rangeof movement when the mass is in the midportion of its range of movementrelative to said first mentioned means, and means for applying a forceto the mass for accelerating the mass in a direction to limit therequired displacement of the support.

7. A gravity meter including, a support, a mass mounted thereon formovement in response to variations in the force of gravity, means onsaid support for limiting the movement of the mass relative to thesupport, means for displaying said support, said means being operable toavoid interengagement of the mass with said first mentioned means, tokeep the mass substantially above a predetermined zero position when thesupport is appreciably above a predetermined reference position, and tokeep the mass substantially below the predetermined zero position whenthe support is appreciably below the predetermined reference position,and means for applying a force to the mass for accelerating the mass ina direction to limit the required displacement of the support.

LUCIEN J. B. LA COSTE. ARNOLD ROMBERG.

REFERENCES CITED The following references are of record in the file 01'this patent:

UNITED STATES PATENTS

